Re: [PATCH v2] signal: add procfd_signal() syscall

[Christian Brauner]

On December 1, 2018 11:09:58 AM GMT+13:00, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>On Fri, Nov 30, 2018 at 5:36 PM Andy Lutomirski <luto@xxxxxxxxxx>
>wrote:
>>
>> On Fri, Nov 30, 2018 at 3:41 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> > siginfo_t as it is now still has a number of other downsides, and
>Andy in
>> > particular didn't like the idea of having three new variants on x86
>> > (depending on how you count). His alternative suggestion of having
>> > a single syscall entry point that takes a 'signfo_t __user *' but
>interprets
>> > it as compat_siginfo depending on
>in_compat_syscall()/in_x32_syscall()
>> > should work correctly, but feels wrong to me, or at least
>inconsistent
>> > with how we do this elsewhere.
>>
>> If everyone else is okay with it, I can get on board with three
>> variants on x86.  What I can't get on board with is *five* variants
>on
>> x86, which would be:
>>
>> procfd_signal via int80 / the 32-bit vDSO: the ia32 structure
>>
>> syscall64 with nr == 335 (presumably): 64-bit
>
>These seem unavoidable
>
>> syscall64 with nr == 548 | 0x40000000: x32
>>
>> syscall64 with nr == 548: 64-bit entry but in_compat_syscall() ==
>> true, behavior is arbitrary
>>
>> syscall64 with nr == 335 | 0x40000000: x32 entry, but
>> in_compat_syscall() == false, behavior is arbitrary
>
>Am I misreading the code? The way I understand it, setting the
>0x40000000 bit means that both in_compat_syscall() and
>in_x32_syscall become() true, based on
>
>static inline bool in_x32_syscall(void)
>{
>#ifdef CONFIG_X86_X32_ABI
>        if (task_pt_regs(current)->orig_ax & __X32_SYSCALL_BIT)
>                return true;
>#endif
>        return false;
>}
>
>The '548 | 0x40000000' part seems to be the only sensible
>way to handle x32 here. What exactly would you propose to
>avoid defining the other entry points?
>
>> This mess isn't really Christian's fault -- it's been there for a
>> while, but it's awful and I don't think we want to perpetuate it.
>
>I'm not convinced that not assigning an x32 syscall number
>improves the situation, it just means that we now have one
>syscall that behaves completely differently from all others,
>in that the x32 version requires being called through a
>SYSCALL_DEFINE() entry point rather than a
>COMPAT_SYSCALL_DEFINE() one, and we have to
>add more complexity to the copy_siginfo_from_user()
>implementation to duplicate the hack that exists in
>copy_siginfo_from_user32().
>
>Of course, the nicest option would be to completely remove
>x32 so we can stop worrying about it.

One humble point I would like to make is that what I care about most is a sensible way forward without having to redo essential parts of how syscalls work.
I don't want to introduce a sane, small syscall that ends up breaking all over the place because we decided to fix past mistakes that technically have nothing to do with the patch itself.
However, I do sympathize and understand these concerns.